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Discussion Starter · #1 · (Edited)
Hi! Seeking advice on what might be causing this click and what a top-end rebuild might entail.

I've been riding my first bike, a 1981 CM400C, for the past 9 months and I'm loving it! It always had a slight "click" to the top-end of the engine, which I read was pretty normal for these engines. It never sounded bad until about month 8. I noticed that the click seemed to getting louder over time, so I stopped riding and did some detective work.

Both pistons had an obvious high clicking sound, but the left piston was much louder. When you get down next to the cam cover on that side it sounds like it's coming from right behind (and maybe a bit above) the exhaust flange. This click follows right along with the RPM of the engine, and does seem to have a bit of "resonance" that happens between 4k-6k RPM, as if something may be loose or something. Up at that speed it sounds pretty freaking bad.

The sound seems to be coming from where the valve tappets would be, so, after a general internet forum + biker buddy consensus, I performed my first (and second) valve adjustment, a cam chain adjustment, and a replacement of both spark plugs. I made all adjustments/gaps to spec from the shop manual, and it really cleaned up the overall sound of the engine. The "bad" left-hand click is still there though, so I'm seeking advice on what my next steps should be.

Another couple things worth mentioning...

The engine fires right up without choke or anything (Texas heat) and it has no problems idling. I don't even think it's suffered a loss of power.

When I first fire up the engine (or at really low idle) the clicking is basically inaudible. I just fooled myself into thinking that it had gone away when I cranked the engine, but after revving the engine and letting it heat up, the clicking became more audible. It seems to get louder/worse at higher RPM.

If I pull the spark plugs while idling, each has a different effect. If I pull the left hand plug, the engine will clearly lose power but it will keep running with an expected amount of trouble. If I pull the right hand plug the engine dies almost immediately.

I made two youtube videos of before and after my initial adjustments. In the second half of the videos you can clearly hear the bad click on the left side:

Before adjustment:

After adjustment:

The difference is subtle, but it's there.

What could the bad click be? I'm thinking that perhaps it's a worn rocker arm / cam shaft, or perhaps a bad valve part. If anyone knows what it might be, I'm all ears.

I'm thinking that a top-end rebuild may be inevitable, but I've never gotten that technical with an engine before. I've watched a bunch of videos and read a lot of material, but I'm wondering if there are any things I should keep in mind before starting. I'm not worried about disassembly or reassembly so much, but I'm definitely concerned that perhaps there will be some fine-tuning along the way that I may or may not be capable of performing myself with basic hand tools. A brief overview of the process would be much appreciated, and any advice regarding lubrication, seals, pitfalls, tools, etc. would be awesome.

Thanks! Love this forum!
 

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Discussion Starter · #3 · (Edited)
Have you adjusted the balancer chain?

If I'm thinking of the correct part, the answer is no. I performed the easy-breezy cam chain adjustment (the 10mm external bolt between the two cylinders underneath the carbs). I did not pull off the crank case cover and do the adjustment shown here:

400Balancer009.jpg


I assume I should try this?

Thanks
 

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Discussion Starter · #4 · (Edited)
UPDATE: Okay, I performed the balancer chain adjustment. I unscrewed the little silver-dollar sized cover off of the crank case cover, loosened the 8mm adjustment nut, and it didn't do anything. I used a flathead to give the flange a little bump and it reset itself. I gave it a bit of pressure on both sides (up and down) and it automatically reset itself every time with a little headroom every time. The little dot on the main shaft bolt moved from about 2 o'clock to 1 o'clock, so according to the manual that's still good. I'm assuming this is how it's supposed to work and that that means the mechanism is still good.

I fired it up and the sound is still there. Here's a video at higher RPM for reference. Beware...nasty


 

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Yep, that's the way it works.


Sounds to me like you may have an exhaust leak. Might check the seals where the downpipes connect to the heads.
 
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